Microchip fabrication involves the formation of integrated circuits (ICs), on a semiconducting substrate. A large number of semiconductor devices or ICs are typically constructed on a wafer formed on a monolithic substrate of a single crystal silicon material. The semiconductor devices are formed by various processes such as doping and patterning the substrate and depositing various conducting or insulating layers of material on the substrate.
As silicon technology advances to ultra-large scale integration (ULSI), the devices on silicon wafers shrink to sub-micron dimension and the circuit density increases to several million transistors per die. In order to accomplish this high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the geometry of various features. In order to achieve this geometry, conformal deposition processes are required.
As an example, the formation of contact/via holes in a semiconductor structure requires the conformal deposition of a film into a relatively deep trench or well-like structure. A film deposited into this structure with poor step coverage may adversely affect the completed semiconductor devices. Insufficient step coverage can lead to unreliable, high leakage and high resistivity contacts. This problem is magnified for ULSI applications where sub-micron high aspect ratio contact/via holes are required.
One deposition process that provides good step coverage in semiconductor manufacture is chemical vapor deposition (CVD). With CVD, gases for forming a film are mixed and reacted in a deposition chamber. The reaction forms the proper film elements or molecules in a vapor state. The elements or molecules then deposit on the wafer surface and build up to form a film. CVD reactions require the addition of energy to the system, such as heating the chamber or wafer. Some films require a narrow temperature range or process window for optimal deposition. In general CVD provides conformal step coverage because reactants or reactive intermediates diffuse rapidly along the substrate surface before reacting.
Various methods are known in the art for improving the step coverage of a CVD film. These methods include apparatus such as rotating holders that rotate the wafers at high speed so that the wafer is positioned at many angles to the deposition gas stream. In some systems, a plasma field is created in the reaction chamber to enhance the film deposition. The plasma adds additional energy to the reaction, resulting in more uniform depositions. An energy source for generating the plasma is typically provided by induction radio frequency (RF) waves.
It is also known to use pulsed RF waves to enhance the CVD process. The pulsed RF waves function to periodically energize the plasma above an optimal deposition energy level. Following the RF pulse, the energy of the plasma drops and diffusion of the reactant gases across the substrate occurs more easily. Such RF deposition processes require expensive and complicated equipment and procedures and ar limited to reactions that can be activated by RF energy.
Another prior art CVD process, U.S. Pat. No. 5,011,789 to Burns, uses heat pulses to heat the temperature of a substrate above 1000.degree. C. for 15 to 90 seconds. This cleans the substrate and allows deposition of an epitaxial layer of silicon to begin. A limitation of this process is that it is directed to the epitaxial deposition of silicon and appears not to be suitable to the deposition of other types of films used in semiconductor manufacture.
The present invention is directed to an improved CVD method especially adapted to large scale semiconductor manufacture and to the deposition of a variety of materials. Accordingly it is an object of the present invention to provide an improved CVD process suitable for semiconductor manufacture. It is a further object of the present invention to provide an improved CVD process that is suitable for depositing films characterized by good step coverage in the formation of semiconductor structures. It is a further object of the present invention to provide a CVD process in which rapid thermal pulses are used to alternately deposit and diffuse a deposition gas over and into the deep recesses (i.e. wells, trenches) of a semiconductor structure.